Mechanism of Threshold Switching in Chalcogenide Phase Change Memory Devices

Chalcogenide-based Phase Change Memory (PCM) utilizes electrically initiated, reversible rapid amorphous-to-crystalline phase-change in chalcogenide materials, such as Ge
₂
Sb
₂
Te
₅
(GST). The markedly different crystalline and amorphous state resistances are used as the two logic states in PCM. The typical IV characteristics of PCM. The upper IV branch 1 describes the processes in crystalline phase where the current (reset pulse) generated Joule heat melts the surrounding material. When the current is off, the material rapidly cools down freezing in its amorphous state. After the bias across the amorphous phase grows above the threshold value, Vth, the system idles for a certain 'delay' time tau
_d
and then switches to a low resistive state (branch 3) in a short time tau
_s
. The mechanism of threshold switching in PCM and its dependence on material parameters remains a puzzle.